Question: In chemistry class, the girl to boy ratio is $3$ to $10$. If there are a total of $39$ students, how many girls are there?
Explanation: A ratio of $3$ girls to $10$ boys means that a set of $13$ students will have $3$ girls and $10$ boys. A class of $39$ students has $3$ sets of $13$ students. Because we know that there are $3$ girls in each set of $13$ students, the class must have $3$ groups of $3$ girls each. There is a total of $9$ girls in chemistry class.